Grate type pelletizing method and apparatus



Jan. 28, 1958 E. w. DAVIS $821,469

GRATE TYPE FELLETIZING METHOD AND APPARATUS Filed Feb. 6, 1953 2Sheets-Sheet l I INVENTOR. Hwy/4R0 W DAV/5 A T QENEYs E. w. DAVIS2,821,469

GRATE TYPE PELLETIZ'ING METHOD AND APPARATUS- James, 1958 2 Sheets-Sheet2 Filed Feb. 6, 1953 uvwszvrozc 527146480 l/V. OAV/J BY A 7- TORNE Y:

United States Patent GRATE TYPE PELLETIZING NIETHOD AND APPARATUS EdwardW. Davis, Minneapolis, Minn, assignor to Regents of the University ofMinnesota, Minneapolis, Mil-1111., a corporation of MinnesotaApplication February 6, 1953, Serial No. 335,539

16 Claims. (Cl. 75-5) This invention relates to an improved method andapparatus for pelletizing ores and other pulverulent materials,particularly iron ore. In the beneficiation of some ores it is necessaryto reduce the ore, as it occurs in nature, .to a fine powder so as torelease the ore particles from the particles of silica and othergangue-like constituents of the ore to thereby permit beneficiation.After being so reduced the fine particles of ore are separated from thesilica and gangue by processes well known in the art and the ore whichis accordingly beneficiated is thereby caused to contain much higherpercentage of iron than as found in nature. The division of the ore intofine particles, though necessary for the beneficiation or concentratingsteps, may be a distinct disadvantage insofar as the use of the productis concerned, for the fine particles are not suitable for use in blastfurnaces of usual construction. For blast furnace use the ore should bein relatively larger particles, thus inch to 1 inch or even larger so asto be capable of packing loosely to allow the furnace blast to passupwardly therethrough during the smelting operation.

It is possible by known procedures to produce balls of ore of suflicientwet strength to permit a limited amount of handling. Thus, the finelydivided ore may be introduced with 8% to 12% of water into a revolvingtube set at a slight gradiant. The particles of ore rolling uponthemselves agglomerate into spherical masses ranging in size from inchup to 1 inch or even larger. By suitable control of the moisture, speedof rotation of the mill and other factors, the production of sphericalor nearly spherical balls or compacts may be accomplished at relativelylow cost. These compacts, however, are not suitable for shipping or foruse directly in the blast furnace because, though strong enough to standsome handling, they are relatively friable when they dry out. It isknown that when such spherical compacts of ore particles are heated to atemperature just below the melting temperature of the ore, the particleswill sinter together and form hard, very strong pellets capable ofshipment, storage and handling and sufliciently strong for use in ablast furnace. To accomplish this, the temperature to which the ballsare heated must be closely controlled. There are many suggestions in thepatented and published arts concerning the production of sinter, butsatisfactory commercial processes for the production of sinteredspherical masses of iron ore at commercially acceptable costs using achain grate stoker mechanism were not accomplished prior to the presentinvention.

1 have discovered that if the moisture containing balls of ore areheated in a thick layer by heat passed upwardly through them, moisturedriven out of the balls at a lower level may condense on and soften theballs at a higher level, with the result that flattening of the ballswill occur, and even though the flattened balls may later on be sintered into hard masses, this flattening is a disadvantage. Even a smallamount of flattening radically reduces the voids between the balls,through which gases must pass, and such flattening increases theresistance and de- 2,821,469 Patented Jan. 28, 1958 use in a blastfurnace.

It is another object of the invention to provide an updraft stoker typepelletizing method and apparatus capable of heat treating sphericalagglomerates of iron ore, uniformly, under accurate control,economically and upon a vast tonnage basis.

It is another object of the invention to provide a method of burningballed ore-fuel compacts in successive relatively thin layers and underclosely controlled conditions which may be varied as needed to effectcontrol and to provide a method whereby the burning of the compacts canbe efiected at a lower overall fuel cost than heretofore possible andwithout undue flattening.

It is another object of the invention to provide an up-draft stoker typemethod and apparatus for the production of merchantable sinteredspherical balls of iron ore of uniform treatment.

Other and further objects of the invention are those inherent in theapparatus herein illustrated, described and claimed.

The invention is illustrated with reference to the drawings in whichcorresponding numerals refer to the same parts and in which Figure l isa perspective view showing in longitudinal section certain portions ofan exemplary apparatus of the present invention;

Figure 2 is a transverse sectional view along the line and in thedirection of arrows 2-2 of Figure 1;

Figure 3 is a plan view of one part of the side edge of the apparatusshown in Figures 1 and 2, the view in Figure 2 being taken along theline and in the direction of arrows 33 of Figure 2.

The method of the present invention will be illus' trated with referenceto the exemplary apparatus herein described and shown in the drawings.Referring to the drawings, the apparatus comprises an endless chaingrate generally designated 10 which is arranged so as to travelhorizontally or nearly so upon two rollers 11 and 12 supported,respectively, on the shafts 11A and 12A. The shaft 11A and roller 11 aredriven by a power source, preferably of the variable speed type, so asto move the upper surface 10A of the chain grate in the direction ofarrow 14. After traveling over the drive roll 11, the chain belt returnsalong the course of travel 103 and after passing over the idler roller12 again resumes its travel along course 10A. The chain grate may be ofsuitable commercial construction. The area of the chain grate to aconsiderable extent determines the capacity of the installation and canbe made as large as mechanical design factors permit. The speed oftravel of the grate is to a considerable degree determined by the rateof burning upwardly through the bed or layer of unsintered balls and bythe thickness of the burden on the grate. Thus, a grate having a widthof 15 feet and a length of the burning zone (denoted by the dimensionBZ) of 15 feet, may have an overall length of 25 feet. Such a machinehaving a 24 inch depth of burden, wherein the grate travels at a rateof, for example, 7 /2 inches per minute, has a capacity in theneighborhood of 1000 tons of finished sintered balls for each 24 hoursof operation.

Along the upper course of travel 10A of the grate are arranged a numberof instrumentalities as follows: Thus, just'after the chain comes aroundthe roller 12 and begins to move along its course of straight travel 10,it travels under the feed box which is provided for depositing a hearthlayer on the grate. The hearth layer preferably consists of alreadyburned balls (which may be be cracked or broken balls) which areseparated after the end of the process. These are relatively hard andresistant and are deposited in a smooth and uniform layer preferablyranging from 1 to 4 inches in thickness, for example 3 inches inthickness. I prefer to use burned balls or chips ranging in size from A2to ,4; inch and these form a pretective layer for the grate which notonly saves the grate surface but also serves as a layer of materialwhich can be charged with heat to assist in ignition.

Adjacent the hopper 15 is another hopper 16 which contains compacts ofspherical or nearly spherical configuration, preferably ranging from A;to inch in size, although larger or smaller sizes, such as 1 inch orlarger, may be used. These compacts may be in the green condition, thatis to say, unburned and containing moisture,

or this layer may be formed of any sintering mixture. I I

prefer to use already burned and broken compacts for this layer. Suchcompacts (whether already burned or green") are coated on this outersurface or mixed as in sintering with pulverized fuel. A binder such asbentonite may be used if necessary. Thus, pulverized coal containingpounds per ton of bentonitc as a binder forms a coating which can becoated onto the balls by tumbling or rolling. Enough pulverized orcrushed fuel is provided to give a fuel content of 5% to 20% based uponthe Weight of the compact, thus 11% may be used. 1

It may be stated that the crushed or pulverant fuel is of carbonaceouscharacter, thus anthracite or bituminous coals, pulverizedsub-bituminous coals, coke breeze, lignite or lignite char, all incrushed or pulverant form may be used. pulverized fuel (thus /2% to 2%)may be admixed with the pulverized concentrated ore prior to passing itinto the balling mills, wherein the spherical agglomerates are built upto sufficient sizes for use. When thus accomplished, that portion of thefuel is substantially uniformly mixed throughout the entire volume ofthe spherical compacts. Then the remainder of the fuel, sufficient tobring the fuel content up to 5% to 20%, or even higher, is applied as asurface deposit upon or a mixture with the compacts. Where previouslyburned compacts are used, there will not, of course, be any fuel withinthe compacts and in such case the entire fuel content is added as amixture of crushed or pulverized fuel with or without a binder (such asbentonite) is added in the requisite percentage amounts as aforesaid.

In any event, the n'nxture contained within the bin 16 and hereindesignated the ignition layer, contains several percent of fuel. Thus,while the main layers -33 may contain up to 1% to 2% fuel (for magnetiteores) the ignition layer 21 may have from 5% to 20% or even more fuel asa surface coating or as a mixture. The reason for the use of increasedfuel in the ignition layer is to foster and encourage the production ofa strong hot ignition layer. It has been discovered in accordance withthis invention that uniformity of product and reduction of overall fuelcosts is dependent to a considerable extent upon strong and uniformignition. In order to insure strength and uniformity of ignition of theentire multiple layered mass which is ultimately deposited upon thegrate as it travels, there is, in accordance with this invention, firstdeposited the ignition layer having a superabundance of fuel. Thethickness of the ignition layer may be from /2 to 3 inches or more, butusually a layer of 1 inch is suificient, particularly wherean'insulative grate layer of previously burned compacts is used. Toanyone versed in the art, it is sufficient to state that a thin, highfuel, sintering layer 21 is placed on top of a comparatively thickhearth layer 20 which is protective in character.

As illustrated there is provided an igniter 22A which may be either gas,oil or coal fired and in the exemplified Where green pellets are used,some of the 3 form is positioned above the ignition layer 21. Ifdesired, however, the ignition may be accomplished by an under grateigniter, although the over-grate igniter is preferred. Beneath theignition zone there is provided a wind box 24 having an outlet 25 towhich suction is applied, when using the over-grate type of igniter.Thus, the windbox 24 may be provided with a negative pressure ofapproximately 1 to 4 inches of water, such as two inches of water. Thesuction box is provided with flanges at its upper edges which serve tosupport the under side of the moving grate sections throughout the areaof the flanges. The suction thus applied pulls the products ofcombustion of the igniter 22A downwardly, against and through theignition layer 21 and into and through the protective and insulativelayer 20. In so doing the fuel content of the ignition layer is ignitedand as the ignition layer 21 emerges from beneath the igniter 22A, ithas been brought to ignition temperature and is in a strongly slowingnd. igni ed i i The pr du io of. a goo hot uniform ignition layer isenhanced by the insulative layer 20 which holds heat, and prevents undueradiation and conduction of heat downward from the under side of layer21.

As the grate then travels throughout the length of the burning zone BZthere are deposited upon it a plurality of layers of spherical or nearlyspherical green compacts which are to be burned. This is an importantfeature of my invention.

I have discovered that if the layer of spherical compacts is too deep,the products of combustion passing upward cool down to such an extentthat moisture condenses on the portions of the layer. This condensedmoisture weakens the spherical or nearly spherical compacts to such anextent that their weight and the superimposed weight of other compactsabove them may cause them to flatten. Even a small amount of flatteningof the spherical or nearly spherical compacts radically reduces thevoids between the compacts through which the gases pass, therebyincreasing the resistance and decreasing the rate of gas flow. Thisradically reduces the burning rate and interferes with the process.Flattening spherical compacts by 17% reduces the percentage of voidsbetween the ball by approximately 50%. The flattened compacts not onlyburn slowly but also cool slowly due to lack of gas flow. I have foundthat when the layer is too deep the spherical compacts, flattened asaforesaid, often discharge from the machine red hot while in other partsof the layer where flattening has not .occurred the compacts will bedischarged cool.

Whether or not moisture condenses out of the gases on the cold sphericalcompacts freshly added on top, or in the upper part of a thick layer, isa complex function of the temperature of the gases, the temperature ofthe surface of the compacts, and the rate of gas flow at'any point.Thus, the rate of flow preferred is the result of blowing about C. F/M.of air (calculated to normal temperature and pressure) upward throughthe grates of the machine for each square foot of grate area. At thisflowing rate and where the compacts have a moisture content of '9 /2%and a diameter of inch (which is normal practice in pelletizing magnetictaconite concentrate), 3-inch layers are a satisfactory thickness, thepreferred range being 2to 5 inches. With drier compacts and otherconditions the same, thicker layers may be used, and with wettercompacts, layers of only 1 inch or 2 inches thick are preferred. Withsmaller compacts, thinner layers are required than with larger compacts.The thickness of the layer of new compacts placed on the surface of thecharge is, therefore, varied directly with the size of the compacts, thetemperature of the top gases and the velocity of these gases, and variesinversely with the moisture in the compacts. Of course, otherrequirements determine workable limits for all of these variables.

The importance of adding the compacts to the surface of the charge in aplurality of layers of proper thickness cannot be over-emphasized. Bythis mode of operation it is possible to build up and burn beds of verygreat total thickness. Five foot thick beds have been built up withoutdifficulty and even thicker beds are possible because the big drop inair pressure through the bed occurs at the comparatively narrow burningzone. The drop in pressure caused by blowing the required amount of airthrough a mass of cold pellets is not large. By adding successive thinlayers of spherical or nearly spherical compacts, the total burden canthus be built up to great depths without undue flattening and only oneignition layer is used, thus reducing overall ignition cost andincreasing process efficiency.

In the exemplary form of apparatus there are accordingly deposited fourlayers by feeder belts, herein designated shuttle belt feeders 26, 27,28 and 29. Obviously, a greater number of layers may be used. These beltfeeders 26 29 are continuous belts and each travels over suitable driveand idler rollers of which the idler rollers 26A-29A are illustrated.The shuttle belt feeders are supported upon a suitable framework (notillustrated) for movement of the delivery ends of the belts back andforth in straight lines across the width of the chain grate as denotedby the arrows 26B-29B. The shuttle belt feeders have their delivery endssupported, respectively, at a uniform distance above the layer ofcompacts immediately underneath each belt. Thus, the belt 26 has adelivery end slightly lower than the belt 27 and it in turn is slightlylower than the belt 28 and so on. Since the shuttle feeder belts havetheir delivery ends moving back and forth in straight lines transverselyin respect to the chain grate 10, the compacts of ore which areuniformly fed onto each one of the shuttle belt feeders (by apparatusnot illustrated) are carried by the shuttle feeder belts and depositedin line back and forth across the chain grate as the latter moves. Itwill be understood that the back and forth motion of the delivery endsof the shuttle belt feeders is relatively much more rapid than the rateof movement of the chain grate and the pattern of deposition (which isactually a close zigzag pattern) has the effect of building up a layerof green spherical or nearly spherical compacts under each of theshuttle belts. Accordingly, under the shuttle belt 26 there is built upa first layer 30 of compacts and likewise under the shuttle belt 27there is built up a second layer 31, under the belt 28 there is built upa third layer 32 and under the belt 2? there is built up a fourth layer33. The thickness of these layers (which, as previously stated, may befrom 1 inch to 5 inches or more) is dependent upon the rate of feed ofcompacts delivered by each of the shuttle belts as compared with thespeed of the chain grate 10.

Each of the belts 26-29 is provided with an adjustable feeder ofstandard design (not illustrated) by means of which the rate of feed ofthe spherical (or substantially spherical) compacts of ore may beadjusted. The feed of compacts onto each belt is thus separatelyadjustable so that the thickness of each layer delivered by the beltonto the next lower layer on the grate can be adjusted with precision.The desideratum of the adjustment is to have each successive layer thickenough so that the moisture bearing hot products of combustion of thelower layers will be cooled down (and the compacts of the upper layerthus heated), but the cooling down of the products of combustion shouldstop short of that temperature at which the moisture would condense outon the ball-like compacts in amounts suflicient to wet them and permitthem to slump (i. e. flatten). it is best, from the operationstandpoint, to not cool the gases much below the dew point. Now thisregulation may be easily achieved simply by regulating the depth oflayer deposited by each belt; the operator can observe the progress ofthe grate. If too much moisture is condensed out the feed to the beltsneed only be decreased, thus thinning the layers deposited. Oralternatively, the moisture in the compact balls may be reduced whileholding the layer thickness constant.

Thus, in an exemplary form of method and apparatus the green andunburned spherical or nearly spherical balls of ore may be deposited inlayers from two to six inches thick, thus, for example, in layers 3 to 4inches thick. The halls range in size from A; to /2 inch in diameter,but may be larger if desired. Thus, balls up to l to 1% inches indiameter may be used but a limit of inch is preferred.

Beyond the point at which layer 33 is deposited there is (for bestresults) preferably provided another hopper 35 in which already burnedcompacts 36 which may be cracked and fragmented compacts similar to thehearth layer separated from previous runs are deposited in a layer 3'2".This layer of material, which requires no burning, serves as aninsulative layer which also prevents upward heat radiation and henceserves to retain the heat generated in layers 3033.

It may be stated in respect to the main layers 353-33 that they containfuel in an amount sufficient to provide heat necessary for firing. Theamount of fuel that is used depends to some extent upon the ore andvaries from 1% to 3%, although more may be used with some ores. Oreswhich contain sulfur or contain a relatively higher percent of the loweroxides of iron, will require a lesser amount of fuel for producingpelletizing temperatures, than ores which do not naturally contain suchmaterials which oxidize during firing and produce some of the heat. Oreswhich are magnetite in character are largely oxidized to hematite duringthe pelletizing operation and in so doing produce heat. Such oresaccordingly require a lesser amount of added fuel for producingrequisite pelletizing temperatures than hematite, which during pelltizing does not reach a state of higher oxidation and hence produces noheat. As as example in firing ore compacts made from beneficiatedtaconite ore, enough heat was produced so that an average coal contentof 1.8% was sufficient.

I prefer to incorporate at least some of the fuel on the green compactsas a surface coating or as a mixture. There are several reasons forthis. In a continuous run some control facility (for regulatingtemperature) is very desirable so as to allow heating up to but notbeyond pelletizing temperatures. If all of the fuel is incorporated intoadmixture with the ore forming the compacts, regulation is muchencumbered, since the compacts which are being burned at any one timemay have been formed hours earlier. However, by putting only part of thepowdered solid fuel (or none of it) into admixture with the ore, thebalance of fuel (or all of it) can be rolled onto the compacts bytumbling and rolling in a balling drum or otherwise mixed immediatelybefore the compacts are delivered by belts 26-29. Hence, there will bevery little time lag between the time the fuel percentage is determinedand the time the compacts bearing the fuel are delivered for burning.Since the stream of powdered fuel delivered to the machine (tube mill)for coating the compacts can be varied readily, this method, which is afeature of this invention, affords Very ready temperature control. Iaccordingly prefer to place some or all of the fuel on the compacts as asurface coating or as a mixture with them. Where the compacts are oflarger sizes (i. e. inch and up) some of the fuel should (for bestresults) be incorporated with the ore forming the compacts.

As shown in Figure 1 there are provided beneath the grate a plurality ofwind boxes 41, 42, 43, 44 and 45 which extend from adjacent the flangeof the suction box 24 throughout the upper course of travel 10A of thegrate to adjacent the delivery end of the grate at roller 11. Each ofthe wind boxes is provided at its upper edge with inturned flanges whichserve as transverse structural members forming supports for the chainlinks that make up the chain grate, and each of the wind boxes isprovided with. a suitable aperture as. at 41A-45A for the introductionof air under pressure. The air under pressure thus introduced into eachof the wind boxes 41-45 is spread under uniform pressure across atransverse area of the grate extending for a prescribed length of thegrate as determined by the distance between the inturned flanges of thewind boxes across the grate. In general, it may be stated that thepressure maintained in boxes 41 through 45 increases in the direction oftravel of the grate. Thus, in an exemplary installation a pressure of 2inches of water was maintained in wind box 41, a pressure of 3 inches ofwater in wind box 42, a pressure of 4 inches of water in wind box 43 anda pressure of 5 inches of water in wind boxes 44 and 45.

It may be stated further, in general, that the wind boxes are locatedbeneath the layers as they are deposited, but slightly downstream in thedirection of travel of the grate from each belt 2649. The final wind box45 is provided for maintaining pressure in the end zone so as toprevent, to some extent, passage of air supplied by the box 44 frommovement endwise under the burning area and thence out through theburned compacts at the delivery end.

Referring to Figures 2 and 3 it is preferred in the present method andapparatus to provide an edge layer on each side of the grate of fuelcoated already burned compacts against which (the side edge of) each ofthe layers through 33 may be deposited. These already burned compactsare preferably mixed with fuel so that they will heat up and thus blockflow of air through them as would otherwise occur if they were cold andnot heated. Thus, adjacent the sides of the grate are provided sidewalls supported by a suitable framework 4-7 and lined with a refractorylayer of brick or other refractory material 48 supported upon thestructure 49. The upper edge of each side wall adjacent each edge of thechain grate is increased in height slightly ahead of the position whereeach of the layers 30 through 33 are deposited. Thus, in the preferredform the side wall steps up as the overall thickness of the burden onthe grate is increased. Adjacent the side wall and preferably throughoutthe width of each of the shuttle belt feeders 26 through 29 there isalso preferably pro vided an internal shield 51 which is close to therefractory wall by the shield 51-52. The shield 51S2 extends from theupper surface of the side wall 46 to a point 54 which is slightly abovethe level of the preceding layer, whether it be the ignition layer 21,as shown in Figure 2, or one of the succeeding layers 33 through 33, asshown in Figure 1. The downstream end of the shield 51 terminates at athin edge 53 and within the space 56 there is situated the delivery end57 of the feed pipe 58 which is connected to an overhead bin notillustrated. Delivered down through the pipe 58 are small pieces orpreviously burned balls, which may be cracked or broken pieces ofpreviously burned balls similar to the hearth layer, if desired, andthese are delivered to a level 59, Figure 2, which is approximatelyequal to the level 6% to which the layer, as layer 30, is built up bythe shuttle feed belt here illustrated as belt 26. It will beunderstood, therefore, that against the refractory wall 48 there iscontinuously deposited a wall of already burned balls and within thiswall and against the plate 51 there is continuously being built up thelayer, as layer 30, the ends of that layer in the transverse directionwith respect to the grate being thus separated from the refractory wall48 by the thickness of the protective and insulative filling ofpreviously burned balls in the space 56. Two similar walls of alreadyburned balls coated with fuel are deposited on each side of the chaingrate at the transverse ends of the paths of movement of each of theshuttle feeder belts 26 through 29, and accordingly each of the layers30 through 33 is protected at its transverse edges from scrapingmovement 8 along the refractory wall 48. In this way the. unburned ballsat' the edges of layers 30-33. are insulated from the cold refractorysurfacev and are. mechanically protected from. abrasion. At the sametime burning of fuel in the mixture on the balls in the spaces 56insures that the air from, the wind boxes will not be channeled throughsuch spaces.

In operation the hearth layer 15 of already burned compacts is depositedfrom the hopper 15 and forms the henrth layer 2.0, upon. which there isthen deposited from the hopper 16 the ignition layer 21, which aspreviously described may be green compacts but is preferably composed ofalready burned compacts containing several percent more fuel than iscontained in the compacts of layers. 30 through 33 (such as 5% to 20% ormore). Thus deposited, the ignition layer passes under (or over) anigniter, here illustrated as the overhead igniter 22A, and becomesignited and reaches a glowing temperature throughout its entire width.There is then deposited in succession from the shuttle feed belts 26through 29 a plurality of layers 39 through 33, preferably protectedfrom the side walls of the grate by the refractory and insulative. layerof previously burned balls and. fuel, as. 5656,. Figure 2. Burning isinitiated at the bottom of layer 30 and as the grate proceeds, the zoneof burning gradually passes up through layer 30 and reaches the surfaceof that layer at approximately the edge 31A of the layer 31. Then as thelayer 31 is deposited, the burning proceeds upwardly through it andreaches the surface at approximately the edge 32A of the layer 32 whichis being deposited on the lower layers. Burning continues to proceedupwardly through layer 32 and reaches the surface at about the edge 33A,whereupon the final layer 33 is deposited. The burning proceeds upthrough the layer 33 and reaches the surface at or approximately at theplace where the protective and insulative layer 37 is deposited, andthen burns to completion in the zone beyond the hopper 35 in the.direction of grate travel. It may be stated parenthetically that the airisforced upwardly from the wind boxes 41 through 45 and as the airpasses upwardly through the gate it meets with relatively littleresistance in those lowermost regions of the superimposed grate loadthat have already burned out and cooled off. Thus, the air that is beingforced up through the load on the grate remains relatively cool (andunexpanded) until it approaches the burning zone at that particularplace. The burning zone, as previously described, gradually slants upthrough the successive layers in the direction of grate travel. At theburning zone, the air which is being forced up through the grate burden,enters into combustion with the solid fuel of the balls and in so doingthe gaseous products of combustion are, like the balls of ore, intenselyheated. Accordingly, in the burning zone there is produced a very rapidand marked expansion of the combustion air and products of combustion,and it is in the burning zone that the movement of the gaseousconstituents encounters greatest resistance to upward travel. Above thecombustion zone the products of combustion in traveling upward give uptheir heat to the superimposed compacts which are green and evensomewhat moist as the surface is reached. In so doing the superimposedunburned ore compacts cool off the products of combustion which areaccordingly delivered to atmosphere at reduced temperatures. Thiscooling oif should not proceed to the place at which moisture condensesout in appreciable amounts on the balls, or if it does the superimposedlayer should not be thick. In this way undue flattening is avoided. Iprefer to operate so that the thickness of the superimposed added layersis such that the moisture bearing products of combustion from lowerlayers is cooled down just short of a temperature at which the moisturebegins to condense out on the balls of the uppermost part of the upperlayer. In this way a maximum recovery of heat is achieved, but themoisture never condenses since it does not reach the dew point.

Accordingly, it will be understood that there is a zone of combustiongradually slanting up through the burden on the grate as denotedgenerally by the dotted line 6565, at which the air rising from thepressure boxes 41 through 45 meets with a maximum resistance to up wardtravel, while below such line there is a relatively lower resistance.Accordingly, toward the delivery end of the chain grate there is atendency for the air to move endwise of the grate and in a downstreamdirection as denoted by the arrow 66. The pressure box 45 and otherpressure boxes which may be provided in the downstream zone, introduceair under pressure into the burden on the grate which therefore producesa pressure gradiant against which the air moving upwardly from the box44 is held. Thus, the air delivered by the box 45 is effective forholding the air from box 44 in place from endwise movement out throughthe end edge of the grate burden. In this way a suflicient upward flowof air is maintained to insure completion of combustion of the unburnedcompacts on the grate.

The insulative layer deposited on the bottom of the grate andinsulative-heating layer preferably deposited on the side edges and theinsulative layer deposited on the top serve to retain in place the heatwhich would otherwise be dissipated to the side walls and radiatedupwardly from the top layer on the grate. In addition, the side walllayers serve to minimize abrasion of the compacts, which would otherwiseoccur before they reach maturity and hardness.

The specific form of apparatus herein described with reference to whichcertain constants of operation, di mensional size, etc. have been givenand the specific sizes of compacts, fuel percentages, wind pressures,etc. are all exemplary and are cited for the purpose of illustrating theinvention, but not by way of limitation thereon.

As many apparently widely different embodiments of this invention may bemade without departing from the spirit and scope thereof, it is to beunderstood that I do not limit myself to the specific embodimentsherein.

What I claim is:

1. The method of burning substantially spherical ore compacts withoutundue flattening which comprises depositing fuel-containing ore compactsin a layer of substantially uniform thickness on the upper surface of amoving grate, igniting said layer along a transverse line across thegrate as the layer is moved thereon, then depositing successively aplurality of further layers of fuelcontaining compacts, blowing airupwardly through those portions of the grate upon which said successiveplurality of layers are deposited for combustion of the fuel in saidcompacts, and then removing all of the compacts from said grate.

2. The method of burning substantially spherical compacts of ore whichcomprises coating compacts with 'solid powdered carbonaceous fuel,depositing said fuel-containing compacts in a substantially uniformignition layer on the upper surface of a moving grate, igniting saidignition layer as the grate moves, depositing on the ignition layer onsaid grate successively, a plurality of further layers containing a lessamount of fuel than that of the compacts of the ignition layer, blowingair upwardly through those portions of the grate upon which saidsuccessive plurality of layers are deposited, said air being blown atpressures that are increased in the direction of travel of the grate andcontinuing the blowing of air under pressure through the layers on saidgrate beyond the place where burning of the compacts has proceeded tothe upper surface of the uppermost layer.

3. The method of burning compacts of ore containing solid fuel whichcomprises depositing upon a moving grate a layer of already burnedcompacts, said compacts having a surface coating of solid carbonaceousfuel, igniting said layer of fuel coated compacts as the grate with thecompacts thereon moves along, then depositing in a plurality ofsuccessive layers unburned compacts of ore containing solid carbonaceousfuel, blowing air under pressure upwardly through those portions of thegrate having the successively deposited layers thereon as the gratemoves along and then removing all of the compacts from the grate.

4. The method of burning or compacts which comprises depositingfuel-containing compacts in a layer of substantially uniform thicknesson the upper surface of a moving grate, igniting said layer along atransverse line across the grate as the layer is moved thereon, thendepositing successively a plurality of further layers of fuel containingcompacts, depositing a final layer of already burned compacts on theuppermost layer on the grate, blowing air upwardly through thoseportions of the grate upon which said successive plurality of layers ofcompacts are deposited for combustion of the fuel in said compacts, andthen removing all of the compacts from said grate.

5. The method of burning compacts of ore containing solid fuel whichcomprises depositing upon a moving grate a layer of already burnedcompacts, said compacts having a surface coating of solid carbonaceousfuel, igniting said layer of fuel coated compacts as the grate with thecompacts thereon moves along, then depositing in a plurality ofsuccessive layers unburned compacts of ore containing solid carbonaceousfuel and depositing a final layer of already burned compacts on theuppermost layer on said grate, blowing air under pressure upwardlythrough those portions of the grate having the successively depositedlayers thereon as the grate moves along, said pressure being increasedin the direction of grate travel and finally removing all of thecompacts from the grate.

6. The method of burning compacts of iron ore containing solid fuel,said compacts containing from 1% to 3% of solid carbonaceous fuel, basedupon the weight of the iron ore thereof, which comprises depositing upona moving grate a layer of already burned compacts having a surface layerof solid carbonaceous fuel thereon, said solid carbonaceous fuel beingin an amount ranging from 3% to 10%, based upon the weight of the burnedcompacts, igniting said layer of fuel coated already burned compacts asthe grate with the compacts thereon moves along, then depositing in aplurality of successive layers said unburned compacts of ore containingsolid carbonaceous fuel in an amount ranging from 1% to 3%, blowing airunder pressure upwardly through those portions of the traveling gratehaving the successively deposited layers of unburned compacts thereon asthe grate moves along, and then removing all of the compacts from thegrate.

7. The method of burning compacts of iron ore containing a minorpercentage of solid fuel therein which comprises depositing an ignitionlayer of fuel containing compacts uniformly upon the upper surface of amoving grate, igniting said ignition layer uniformly as it moves, atopposite edges of the grate depositing walls of already burned compactsand between said walls depositing successive layers of unburned compactscontaining fuel, blowing air upwardly through the layers of compacts onthe grate until the compacts have burned completely through and removingthe burned compacts from the grate.

8. The method of claim 7 further characterized in that the unburnedcompacts deposited as walls at opposite edges of the grate aresurface-coated with solid carbonaceous fuel.

9. The method of claim 7 further characterized in that the ignitionlayer of compacts which are already burned compacts are coated withsolid carbonaceous fuel in an amount ranging from 5% to 20% of theweight of the compacts and the green compacts have a fuel content of 1%to 3% solid carbonaceous fuel.

10. The method of claim 7 further characterized in that a grate layer ofburned compacts is first deposited. upon the upper surface. of the gratebefore there is deposited thereon the first fuel containing layer ofcompacts.

11. The method of claim 7 further characterized in that a final layer ofburned compacts is. deposited, upon the uppermost layer of compacts onsaid grate.

12. In the pelletizing of ball-like. moisture containing compacts ofiron ore according to an updraft grate-typepelletizing process. whereinthe moisture of the compacts is evaporated and they are burned to.sintering temperatures, the improvement which comprises; depositing saidcompacts on a layer of fuel-containing ore compacts. on said grate in amultiplicity of successive. layers, igniting the fuel layer and forcingair under pressure upwardly through said superimposed layers to, carrythe heated products of combustion through the layers. of compacts, saidpressure increasing in the direction of travel of the grate whereby themoisture evaporated from compacts in lower layers does not materiallycondense upon and increase the moisture of the compacts. of superimposedadded layers.

13. The method of burning iron ore compacts which comprises formingcompacts of finely divided ore and solid fuel, the. proportion of solidfuel being substantially in excess of that required, for maintainingsintering temperature of the, compacts, depositing said. compacts in alayer upon a traveling grate, igniting said layer,

depositing upon said layer another layer of compacts containingsubstantially less powdered fuel incorporated therein than in said firstlayer, blowing air up through the layers on said grate until all thecompacts have burned out.

14. The method of burning iron ore compacts: which comprises; formingcompacts for a grate layer comprising finely divided iron ore and solidcarbonaceous fuel, the amount of fuel ranging from. 3%v to 10% by weightas compared to the amount of ore, depositing said, grate layer compactsupon the upper surface of a moving grate, igniting said grate layer,depositing: upon said grate layer at least one further layer ofcompacts; containing finely divided pulverized fuel in an, amount.substantially less on a percentage basis than the amount of finelydivided solid fuel in said grate layer, blowing air upwardly throughsaid, layers of compacts on said grate until they have finally burned,through, and removing' the burned compacts from the grate.

1.5. In an. apparatus for burning ore compacts which comprises a chain,grate having side walls therealong, aplurality of. rollers forsupporting said chain gratev for a course of travel in a, generallyhorizontal plane, means for rotating the rollers for moving the gate,means for depositing layers of compacts on the. grate, an igniter, andwind. box means under said grate, the. improvement. which resides inmeans for depositing vertical wall layers of granular materiallongitudinally along the outer edges of the grate adjacent thev insideof each of the side walls thereof, means for simultaneously depositingon the grate other granular material in the spacev between said walllayers as said Wall layers are deposited.

16. The apparatus of claim 15 further characterized in that the grate isprovided with a plurality of stations at each of which means is providedfor depositing adjacent the inside of each, of the side wallslongitudinal wall layers of granular material, and for simultaneouslydepositing at. each station between said wall layers a layer. of othergranular material on. the grate. to a level substantially equal to theheight of said wall layers.

References Cited in the tile of this. patent UNITED STATES PATENTS.

916,397 Dwight Mar. 23, 1909 942,052 Bellinger Dec. 7, 1909 951,199Perkins et al. Mar. 8, 1910 1,221,962 Bittmann Apr. 10, 1917 1,598,176Tharaldsen Aug. 31, 1926 1,684,958 Hyde Sept. 18, 1928 1,896,884 Cooperet al Feb. 7", 1933' 2,052,329 Wendeborn Aug. 25, 1936 2,143,905 AhlmannJan. 17', 1939 2,411,873 Firth Dec. 3, 1946 2,511,400 De Jahn June 13,195.0 2,532,335. Royster Dec. 5, 1950 2,608,481 Roystel'. Aug. 26, 1952FOREIGN PATENTS 229,608 Great Britain Feb. 26, 1925 379,057 GreatBritain Aug. 25,, 1932 498,837 Great Britain Jan. 19, 1939 510,786 GreatBritain Aug, 8, 1939 573,539 Greatv Britain Nov. 26, 1945 645,444 GreatBritain Nov. 1, 1950

1. THE METHOD OF BURNING SUBSTANTIALLY SPHERICAL ORE COMPACTS WITHOUTUNDUE FLATTENING WHICH COMPRISES DEPOSITING FUEL-CONTAINING ORE COMPACTSIN A LAYER OF SUBSTANTIALLY UNIFORM THICKNESS ON THE UPPER SURFACE OF AMOVING GRATE, IGNITING SAID LAYER ALONG A TRANSVERSE LINE ACROSS THEGRATE AS THE LAYER IS MOVED THEREON, THEN DEPOSITING SUCCESSIVELY APLURALITY OF FURTHER LAYERS OF FUELCONTAINING COMPACTS, BLOWING AIRUPWARDLY THROUGH THOSE PORTIONS OF THE GRATE UPON WHICH SAID SUCCESSIVEPLURALITY BY LAYERS ARE DEPOSITED FOR COMBUSTION OF THE FUEL IN SAIDCOMPACTS AND THEN REMOVING ALL OF THE COMPACTS FROM SAID GRATE.
 15. INAN APPARATUS FOR BURNING ORE COMPACTS WHICH COMPRISES A CHAIN GRATEHAVING SIDE WALLS THEREALONG, A PLURALITY OF ROLLERS FOR SUPPORTING SAIDCHAIN GRATE FOR A COURSE OF TRAVEL IN A GENERALLY HORIZONTAL PLANE,MEANS FOR ROTATING THE ROLLERS FOR MOVING THE GATE, MEANS FOR DEPOSITINGLAYERS OF COMPACTS ON THE GRATE, AN IGNITER, AND WIND BOX MEANS UNDERSAID GRATE, THE IMPROVEMENT WHICH RESIDES IN MEANS FOR DEPOSITINGVERTICAL WALL LAYERS OF GRANULAR MATERIAL LOGITUDINALLY ALONG THE OUTEREDGES OF THE GRATE ADJACENT THE INSIDE OF EACH OF THE SIDE WALLSTHEREOF, MEANS FOR SIMULTANEOUSLY DEPOSITING ON THE GRATE OTHER GRANULARMATERIAL IN THE SPACE SAID WALLS LAYERS AS SAID WALL LAYERS AREDEPOSITED.